1. Field of the Invention
This invention relates to the manufacture of glass ribbon by the flotation process.
2. Description of the Prior Art
In the flotation process for manufacturing glass ribbon, a layer of molten glass is poured onto a bath of molten metal contained within a tank. Because the density of the molten glass is less than that of the molten metal, it floats upon the molten metal bath. The glass is caused to advance along the length of the molten metal bath and is permitted to flow freely in a lateral direction so that the thickness of the molten glass is gradually reduced. As the molten glass advances, it is gradually cooled to form a flat glass ribbon floating upon the molten metal bath. Extraction rollers facilitate the forward motion of the glass ribbon and draw the relatively cool glass ribbon from the tank.
The molten metal bath normally consists of tin or a tin alloy. Since the metal bath is oxidized by contact with ambient air, it is shielded by a reducing gas atmosphere made of hydrogen and nitrogen. The protective atmosphere is retained within an enclosure positioned above the tank. The enclosure must have apertures for the admission of the molten glass and for the removal of the glass ribbon. Inevitably, ambient air passes through these apertures and reacts with the bath of molten metal to form oxides. This oxidizing reaction particularly occurs at the upstream area of the bath where the molten metal is relatively hot. The oxides float on the surface of the molten metal bath and are drawn by the glass ribbon to the point where it leaves the tank. As the glass ribbon leaves the tank, the oxides adhere to the lower surface of the glass ribbon. The oxides contaminate the glass ribbon thereby adversely affecting the quality of the manufactured glass.
To alleviate this oxide problem, various prior art processes have been developed. In a prior art process, molten metal is withdrawn from the downstream end of the tank and is mixed with hydrogen gas to reduce the metal and thereby decrease the number of oxides. The metal is then heated and reintroduced into the upstream end of the tank. However, the metal extracted from the downstream end of the tank is at a relatively cool temperature of 650.degree. C. which is unsuitable for reduction reactions.
There are additional problems present in the flotation process. In recent years, the output for flotation glass plants has risen to 600 tons of glass ribbon per day. As the enormous amount of hot molten glass used to produce these large outputs is poured onto the molten metal bath, a large amount heat is transferred from the molten glass to the bath. The central region of the bath becomes considerably hotter than the side regions of the bath. It is desirable to reduce the thermal gradient thus formed across the molten metal bath to ensure that the glass ribbon floating upon the molten metal bath is free from transverse variations in its thickness.
In a prior art process, molten metal is caused to flow underneath the glass ribbon from the center of the bath toward each side of the molten metal bath. Although the thermal gradient is somewhat decreased by this process, it is never eliminated. The central regions remain undesirably hotter than the side regions of the molten metal bath. Further, although a continuously renewed molten metal surface contacts the lower surface of the glass ribbon, the molten metal is not reduced because the glass ribbon separates the protective reducing atmosphere from the surface of the circulating molten metal. Thus the number of oxides contaminating the metal bath are not significantly decreased.